The invention relates to a hybrid drive system for a vehicle having a generator, which is coupled to an internal combustion engine and is connected to an electric drive motor via a generator converter, a voltage intermediate circuit and a motor converter, with an energy store being connected to the voltage intermediate circuit.
The invention also relates to a method for controlling a hybrid drive system such as this.
A system such as this, as well as a method such as this, are known from DE 101 37 908 A1.
A hybrid drive system is used both for road vehicles and for rail vehicles. A battery or a high-power capacitor may be used as the energy store.
Because its energy density is relatively low, the no-load voltage of a capacitor falls relatively quickly as it is discharged. In the case of a battery, the no-load voltage depends on various factors, such as the state of discharge, the previous history and the temperature. In the case of a capacitor or in the case of a battery, the terminal voltage of the energy store is also governed, in addition to the no-load voltage, by the load-dependent voltage drop across its own internal resistance. This then leads to a low intermediate-circuit voltage, which has a negative effect on the operation of the drive motor and of the generator.
It is already known for this disadvantage of the low voltage to be counteracted by producing a high current flow, although this requires heavy, oversize components. It is also already known for an energy store to be connected to the intermediate circuit via a step-up/step-down controller. However, this requires complex, expensive electronics. Finally, it is known from German Patent Application 101 37 908 that the energy store can be connected to the intermediate circuit via a switch. When it is not required, the energy store can then be disconnected from the rest of the system, so that the low voltage on the energy store cannot disadvantageously affect the drive.
In this case, however, it is accepted that no more power can be drawn from the store once the energy store has been disconnected. Particularly if the store is a high-power capacitor, its capacitance then cannot be completely used. In the end this leads to the internal combustion engine and the generator having to be made larger.